The present invention concerns fuel injection valves.
German published Patent Application No. 195 38 791 concerns a fuel injection valve for fuel injection systems for internal combustion engines in which the valve closing body is actuated by a piezoelectric actuator. The piezoelectric actuator has a plurality of piezo layers made of a piezoelectric material. Electrodes are arranged between the piezo layers so that an electrical voltage can be applied to the piezo layers, causing the piezoelectric actuator, used for actuating the valve closing body, to expand.
A problem with using piezoelectric actuators is believed to be thermal expansion. Piezoelectric materials, unlike materials such as steel or plastic, have a negative temperature expansion coefficient. Therefore, the piezoelectric actuator contracts with increasing temperature, while the surrounding housing expands. The different temperature expansion coefficients of the piezoelectric actuator and the housing result in a temperature-dependent valve lift if not compensated using appropriate measures.
For temperature compensation, German Published Patent Application No. 195 38 791 apparently proposes that the valve housing be designed as two parts made of two different materials. For example, it is proposed that one housing part be made of steel and the other housing part be made of Invar. By an appropriate selection of the length of the first housing part made of steel and of the second housing part made of Invar, the overall thermal expansion of the housing should be matched to the thermal expansion of the piezoelectric actuator and thus the piezoelectric actuator and the housing surrounding the piezoelectric actuator expand and contract in the same manner.
It is believed that a disadvantage of this measure is the cost of manufacturing the valve housing and the relatively high cost of the material of the second housing part, which is preferably made of Invar. Furthermore, it must be taken into consideration that the valve housing and the actuator may be of different temperatures. Thus the piezoelectric actuator may heat up due to its heat losses, in particular, when the fuel injection valve is frequently actuated, and its temperature is only slowly transferred to the valve housing. On the other hand, the temperature of the valve housing is influenced by the heat transferred from the internal combustion engine on which the fuel injection valve is mounted. This type of temperature compensation is therefore not believed to be satisfactory.
German Patent No. 195 19 192 purportedly concerns a hydraulic lift transformer arranged between the piezoelectric actuator and the valve needle that actuates the valve closing body. Temperature compensation results from the fact that the lift transformer only responds to the relatively quick movement resulting in the intended opening of the fuel injection valve, whereas a relativity slow, temperature-dependent expansion or contraction of the piezoelectric actuator may cause the hydraulic fluid to leak out via guide gaps. It is believed that a disadvantage of this design is, however, the relatively high cost of the hydraulic lift transformer.
Other temperature compensation methods include forced tempering of the piezoelectric actuator using a liquid or gaseous medium, which is held at a constant temperature in a closed circuit, or a series arrangement of piezoelectric actuators with a temperature-compensating piece, which is arranged between the piezoelectric actuator and a valve needle actuating the valve closing body, for example. While the first method may be relatively costly, the use of a compensating piece arranged in series is believed to have the disadvantage that the piezoelectric actuator and the compensating piece, as mentioned before, are not necessarily subjected to the same temperature or the same temperature variation; therefore, temperature compensation is relatively inaccurate.
The fuel injection valve according to an exemplary embodiment of the present invention is believed to have the advantage that the piezoelectric actuator of the fuel injection valve has considerably improved temperature compensation. According to an exemplary embodiment of the present invention, one or more temperature compensation layers having a temperature expansion coefficient with opposite signs with respect to the temperature expansion coefficient of the piezo layers are provided directly in the piezoelectric actuator. Through an appropriate selection of the number and thickness of the temperature compensation layers, accurate temperature compensation can be achieved.
By embedding the temperature compensation layers in the piezo layers of the piezoelectric actuator, it is at least better ensured that the temperature compensation layers are subjected to the same temperature and the same temperature variation as the piezo layers of the actuator. In particular, a large contact surface exists between the piezo layers and temperature compensation layers, so that the temperature of the temperature compensation layers and that of the piezo layers are quickly equalized. This is important because the heat loss of the piezoelectric actuator can be subjected to considerable fluctuation when the actuation frequency of the fuel injection valve varies as a result of a variation in the internal combustion engine speed. Due to the large contact surface between the temperature compensating layers and the piezo layers and the proximity of the temperature compensating layers to the piezo layers, temperature compensation by the temperature compensation layers can quickly follow these fluctuations. In addition, a change in the temperature of the actuator due to fluctuating amounts of heat transferred from the internal combustion engine can be quickly compensated by using the method according to the present invention. Expensive forced tempering of the piezoelectric actuator is not necessary.
In a particularly advantageous manner, the temperature compensation layers can be used simultaneously as electrodes for activating the piezo layers if the temperature compensation layers are made of a metallic material.
Thus, the piezoelectric actuator is temperature compensated to a high degree. However, the valve housing surrounding the actuator, which is usually made of metal or plastic, can still be subjected to thermal expansion resulting in a temperature-dependent position shift of the valve seat body with respect to the valve closing body connected to the actuator. In order to avoid this, an equalizing sleeve made of a ceramic material is advantageously provided, which either surrounds the piezoelectric actuator or is itself surrounded by the piezoelectric actuator. The piezoelectric actuator is either in contact with the valve housing via the equalizing sleeve or actuates the valve closing body via the equalizing sleeve and, optionally, a valve needle. If the equalizing sleeve has the same axial length as the piezoelectric actuator, the temperature of the valve housing has no effect on the axial position of the valve closing body with respect to the axial position of the valve seat body, i.e., temperature compensation of the valve housing is achieved.
According to an exemplary embodiment, a first end of the piezoelectric actuator is connected to the valve closing body via a valve needle and a first end of the equalizing sleeve is in contact with the valve housing. A connecting element, which may be plate-shaped for example, is held in contact with the second end of the equalizing sleeve and the second end of the piezoelectric actuator by a spring. According to another exemplary embodiment design, the first end of the piezoelectric actuator is in contact with the valve housing, and the first end of the equalizing sleeve is connected to the valve closing body via a valve needle. A plate-shaped connecting element is held in contact with the second end of the equalizing sleeve and the second end of the piezoelectric actuator by a spring also in this case.